As processes for radical polymerization of polymerizable compounds having a polymerizable double bond, such as acrylic acid, methacrylic acid and their derivatives, emulsion polymerization, suspension polymerization, solution polymerization and bulk polymerization are known. Of these processes, emulsion polymerization, suspension polymerization and solution polymerization are carried out by dissolving or dispersing a reactive compound in a reaction solvent so that they are advantageous in that the polymerization temperature can be easily controlled and the reaction solution has flowability even when the conversion (polymerization ratio) is high.
In the emulsion polymerization and the suspension polymerization, however, operations to separate the resulting polymer from a dispersion medium, such as precipitation, filtration, washing and drying, are necessary, so that the process becomes complicated. Moreover, it is difficult to completely remove a dispersant or an emulsifying agent used in the polymerization process from the polymer, so that the residual emulsifying agent or dispersant may adversely affect the properties inherent in the polymer, such as water resistance or strength, and may also exert an undesirable influence on the processability when the polymer is mixed or kneaded with other substances or modified with other substances. The solution polymerization process is advantageous in that the temperature of the polymerization reaction can be easily controlled because a large amount of an organic solvent is used and that designing of a polymer can be relatively easily made because of its homogeneous reaction system. This process, however, has a problem in that there is a great difference in the monomer concentration between the initial stage and the later stage of the polymerization. As a result, a large proportion of a low-molecular weight polymer is produced and the molecular weight distribution is widened. In order to solve the problem, a method of adding a monomer by portions or a method of adding a monomer dropwise has been attempted. By the use of such a method, however, an advantage of the solution polymerization, i.e., simple reaction process, is impaired.
In case of a polymerization reaction associated with chain transfer, the organic solvent used as a reaction solvent in the solution polymerization generally participates in the polymerization reaction such as termination reaction or chain transfer reaction, so that the presence of the organic solvent complicates designing of a polymer.
In addition, the polymer obtained by the solution polymerization contains a large amount of an organic solvent. In the use of the polymer, therefore, it is necessary to remove a large amount of the organic solvent used in the reaction, and as a result, the productivity becomes low. Moreover, if the polymer solution is used outdoors as it is, the organic solvent evaporates and exerts an adverse effect on the natural environment.
In contract therewith, bulk polymerization uses no solvent, so that it is unnecessary to use an organic solvent, a dispersant, an emulsifying agent or the like, and any impurity (e.g., organic solvent) which participates in the polymerization reaction is not contained in the reaction system. Therefore, not only does the reaction system become simplified but also the resulting polymer is free from impurities such as an emulsifying agent and a dispersant. Further, an operation of removing the solvent is unnecessary to obtain the aimed-for polymer. From these viewpoints, bulk polymerization is industrially advantageous.
In the bulk polymerization method, however, the polymerization reaction rate generally is extremely high, and in fact, it is very difficult to control the bulk polymerization reaction. The polymer produced at high temperatures, because of the uncontrollable reaction rate, may have unstable end groups of molecules attributable to disproportionation termination, or the molecular weight of the polymer is lowered. Further, branching or gelation of a polymer is liable to occur due to the hydrogen abstraction from the previously produced polymer. On this account, it becomes difficult to design molecular weight, molecular weight distribution, etc. of the polymer, and besides, designing of a definite molecular structure becomes difficult because of branching of a polymer or production of disproportionation termination terminals. Furthermore, a gel is sometimes produced abruptly and in a large amount, and in the worse case, there is a danger of explosion attributable to a runaway reaction.
Of various monomers, styrene or methyl methacrylate has characteristics of a relatively low polymerization reaction rate, so that its reaction can be controlled even in the case of the bulk polymerization, and methods to control the reaction have been studied for a long time. In the bulk polymerization of styrene or methyl methacrylate, a mercaptan is sometimes used to regulate the molecular weight or the molecular weight distribution.
For example, it is known that, in the bulk polymerization of a styrene type unsaturated compound (e.g., styrene) whose reaction proceeds relatively mildly, a mercaptan is used to control the progress of the bulk polymerization reaction. More specifically, Japanese Patent Publication No. 401/1980 discloses “a polymerization process comprising bringing a polymerizable ethylenically unsaturated monomer into contact with an organic mercaptan having at least one thiol group in the presence of oxygen at a temperature-of about 20° C. to about 200° C. for a period of time sufficient to accomplish substantially complete conversion of the monomer into a polymer”. In this reaction, presence of oxygen is essential, and bulk polymerization of an ethylene type unsaturated monomer is conducted using a mercaptan, which is the only activator employable in the presence of oxygen, in combination with oxygen. Therefore, this reaction does not effectively proceed in an atmosphere wherein oxygen is absent. Example 6 of this publication discloses that methacrylic acid, hydroxypropyl methacrylate, butyl acrylate and styrene are copolymerized using 1-thioglycerol as a mercaptan under temperature conditions of 85° C. to 140° C., substantially 140° C., with introducing air. In the bulk polymerization described in this publication, the mercaptan is used together with oxygen, that is, a mercaptan and oxygen are shown as essential substances to initiate polymerization, and there is no description of the use of a mercaptan alone as a polymerization catalyst for an ethylene type unsaturated compound. However, if the mercaptan described in this publication and oxygen are used as polymerization catalysts, oxygen is positively blown into the reaction system, so that there is great danger of explosion or fire in the production of a polymer in a large plant. Even if the production of a polymer is safely made, there is a problem that the resulting polymer may be colored.
In claims of Japanese Patent No, 2,582,510, an invention of “a-process for producing an acrylic polymer, comprising bulk polymerizing a monomer component containing an acrylic acid monomer as a main ingredient, wherein the polymerization reaction system for the bulk polymerization of the monomer component is in an inert gas atmosphere, contains a mercaptan and substantially contains no polymerization initiator” is disclosed.
It is described in the above publication that one role of the mercaptan in the invention is molecular regulation, i.e., regulation of molecular weight and molecular weight distribution, and another role thereof is to mildly promote bulk polymerization of the monomer component up to a high conversion and to gently control the polymerization reaction rate when the reaction system substantially contains no initiator. Comparative Example 3 described in the paragraph No. 0032 of the above publication is an experimental example wherein the reaction was conducted without using 30 parts of octyl thioglycolate that is a mercaptan used in Example 1. According to this comparative example, only the polymerizable monomer was heated in a nitrogen atmosphere, then the temperature rose up to 130° C. after 90 minutes from the beginning of the polymerization, and as a result, the monomer could not be polymerized stably and the resulting polymer was in a state of a gel. That is, it can be seen from the comparison between Comparative Example 3 and Example 1 that the polymerization reaction is initiated by the heat of the monomer and the role of octyl thioglycolate used as a mercaptan is to regulate the molecular weight of the resulting polymer and to inhibit rapid progress of the bulk polymerization reaction.
In the conventional bulk polymerization reaction of an unsaturated monomer using a mercaptan, the mercaptan is used to inhibit progress of the bulk polymerization reaction which is liable to run away (that is, the mercaptan is used as a negative catalyst), as described above, and any technical idea to use the mercaptan as a reaction catalyst for bulk polymerizing an unsaturated monomer has not been known so far.
The present invention provides a novel catalyst used for bulk polymerizing a polymerizable unsaturated compound.
The invention further provides a novel catalyst for bulk polymerization, by the use of which a high conversion can be achieved even when a polymerizable unsaturated compound having extremely high reactivity, such as an acrylic acid derivative, is bulk polymerized, the dispersion index of a molecular weight of the resulting polymer can be made small, and hydroxyl groups can be introduced into the resulting polymer.
Still further, the invention provides a process for bulk polymerizing a polymerizable unsaturated compound, wherein the bulk polymerization reaction can be certainly controlled by the use of the above-mentioned catalyst.